Polyurethane resins have been used extensively as a useful material in adhesives, coating materials and as resin modifiers. In addition, recently, the use of synthetic resins, either water-soluble or in the form of water-based emulsions, has increased because resins synthesized or solubilized in a nonaqueous solvent system pose environmental and toxicological problems because the solvent is released into the atmosphere. In the field of polyurethane resins, water-soluble polyurethanes and water-based emulsion-type polyurethanes are used increasingly in adhesives, coatings, and other practical applications in place of a conventional polyurethane dissolved in an organic solvent. In addition, new applications for water-based polyurethanes are being investigated extensively. In short, the application of water-based polyurethane resins, both water-solution types and water-emulsion types, is expanding and is expected to continue expanding in the future.
The water-soluble and water-based emulsion polyurethanes in present-day use include emulsions of thermoplastic polyurethanes of relatively high molecular weight, and comprising mainly straight-chain molecular structures. The emulsions are prepared by introducing an anionic, cationic or nonionic hydrophilic group onto the polyurethane, thereby causing self-emulsification or dispersion, or by adding an emulsifier to a hydrophobic polyurethane resin to induce dispersion and emulsification of the resin in water.
In addition, the use of radiation-curable resins, whereby polymerization is induced by various types of radiation, such as ultraviolet ray or electron beam, recently has been expanding quickly, particularly in the coatings industry, because such resins possess the advantageous property of having a low energy requirement for processing, at a low temperature and in a short time. In the art of radiation-curable resins, nonaqueous radiation-curable resins are the typical and conventional product form.
Nonaqueous radiation-curable resins are designed, as explained above, to allow processing with low amounts of energy, at low temperatures, and in a short time. Therefore, 100% of the components of nonaqueous radiation-curable resins are resin components. The conventional nonaqueous resins incorporate a radiation-curable resin of medium molecular weight, and a large quantity of a low molecular weight component, i.e., a monomer, for adjusting the viscosity to improve application properties. The low molecular weight component is termed a reactive thinner monomer, which include, for example, vinyl acetate, vinylpyrrolidone or an alkylacrylic ester.
Conventional water-soluble and water-based polyurethane emulsions, however, have the following disadvantages. The polyurethane emulsions of the prior art are emulsions of a straight-chain thermoplastic polyurethane of relatively high molecular weight. Therefore, for films derived from such prior polyurethane emulsions, performance improvements are presently needed in, for example, heat-stable adhesion, solvent resistance, and chemical resistance. Moreover, to improve various properties of films derived from conventional polyurethane emulsions, attempts have been made to incorporate a variety of crosslinking agents, for example, a melamine, such as trimethylol melamine, an epoxy or a blocked isocyanate, into the composition. As a result of adding crosslinking agents, some performance improvements have been demonstrated, but composition performance still is generally unsatisfactory. In addition, when crosslinking agents are added to the polyurethane emulsion, it is necessary to use a processing temperature that is higher than the activating temperature of the reactive group of the crosslinking agent. Accordingly, an advantage of the conventional polyurethane emulsions, i.e., the ability to form a film which exhibits good physical properties at the drying temperature of the composition, is eliminated.
The heat resistance of the substrates must be considered. This consideration limits the processing conditions and the scope of applications for the polyurethane emulsion. In short, satisfactory polyurethane emulsions have not been developed yet. Deficiencies in polyurethane performance are attributed primarily to the fact that the above-described prior art polyurethane emulsions lack a functional group that can react with a crosslinking agent. In other words, improvements in conventional polyurethane emulsions by the use of crosslinking agents are limited to improvements in physical properties attributed to polymer chain entanglement, like an interpenetrating polymer network (IPN) between the polyurethane resin and the self-polymerization product of the crosslinking agent.
It, therefore, is a prime goal to develop a polyurethane emulsion that retains the advantages of prior art polyurethane emulsions, i.e., an ability to form a film exhibiting their physical properties when treated with sufficient energy merely for drying the composition, and that also has an ability to form a network structure which overcomes the various disadvantages of the prior art polyurethane emulsions. The development and availability of such a polyurethane emulsion is commercially valuable.
With respect to radiation-curable resins, radiation-induced setting resins of the prior art require the addition of 20 to 70 parts by weight of a low molecular weight reactive thinner monomer to the intermediate molecular weight resin component to adjust composition viscosity and film thickness. The addition of a reactive thinner monomer poses problems with respect to safe storage and with respect to using materials that are harmful to the human body and of objectionable odor.
Furthermore, the polymerization of a large amount of a low molecular weight reactive thinner monomer poses the following problems:
(1) A majority of the cured films, or the substrate, exhibit an odor specific to the radiation-curable resin. The odor is attributed primarily to the reactive thinner monomer and is a very troublesome problem. PA1 (2) The excellent film flexibility attributed to the intermediate molecular weight resin is seriously impaired. This results in film brittleness, which is a defective physical property. The film therefore tends to perform badly. PA1 (3) The large degree of shrink observed when the reactive thinner monomer undergoes curing polymerization causes the problem of poor adhesion between the film and the substrate. PA1 (1) After the hydroxyl group-containing compound containing a polymerizable unsaturated group is reacted with the polyisocyanate, a reaction between the residual isocyanate group and the polyol, which requires heating, is needed. PA1 (2) After the emulsion is produced, the solvent is removed under reduced pressure and in the presence of water. During the process of solvent removal, oxygen, which suppresses polymerization of the polymerizable unsaturated group, also is removed. PA1 (1) The reaction between the emulsion of a polyurethane having active amino groups and the isocyanate compound having a polymerizable unsaturated group allows introduction of the polymerizable unsaturated group into the polyurethane. Moreover, this reaction proceeds easily in the presence of water at low temperatures, and therefore the conventional process involving heating at a high temperature is not required. PA1 (2) After the organic solvent used in the preparation of the emulsion of a polyurethane having active amino groups has been removed, the above-mentioned method allows the reaction with the isocyanate compound having a polymerizable unsaturated group to occur in the emulsion or in the presence of water. Thus, the method overcomes a problem of the prior art by eliminating the possibility of polymerizing the polymerizable unsaturated group during production of the water-based urethane acrylate. The method does not require the excessive use of an inhibitor. The method, therefore, is very attractive as an industrial production process. Moreover, the product also has excellent curing properties after application to a substrate. PA1 (1) During preparation of the urethane prepolymer, carboxyl groups are introduced, in advance, into the prepolymer molecule by means of a reaction between a polyol component having a carboxyl group, such as dimethylol propionate, and an organic polyisocyanate. Then, the carboxyl group is neutralized with a basic compound, such as triethylamine, trimethylamine, diethanol monoethylamine, dimethyl ethanolamine, sodium hydroxide or potassium hydroxide, to convert the carboxyl group into a salt of the carboxyl group. PA1 (2) During preparation of the urethane prepolymer, an oxyethylene chain is introduced, in advance, in the prepolymer molecule in an amount of 5% to 20% by weight. Then a nonionic surfactant having a hydrophilic-lipophilic balance value of from 6 to 18 is added to and mixed with the ethoxylated urethane prepolymer at 50.degree. C. or less, after the ethoxylated urethane prepolymer has been prepared. After considering both the emulsification and dispersion properties of the final product and the water resistance of a cured film derived from the final product, the amount of nonionionic surfactant added to the urethane prepolymer preferably is from 2% to 15% by weight, relative to the urethane prepolymer. PA1 (3) After preparation of the urethane prepolymer, the urethane prepolymer is reacted with a water solution of the sodium salt or potassium salt of aminoethanesulfonic acid, aminoacetic acid, or similar amino-substituted organic acid, in an amount corresponding to 5 to 50% of the terminal isocyanate groups of the urethane prepolymer, and preferably to 5 to 30% thereof, at a temperature ranging from 5.degree. C. to 50.degree. C., and preferably from 20.degree. C. to 40.degree. C., for 60 minutes.
To reduce or completely eliminate such disadvantages, it is necessary to develop both chemical and processing solutions to solve these important problems. One simple solution to the problem of using reactive thinner monomers is the proposed addition of water to reduce the amount of the reactive thinner monomer in the composition. The radiation-curable resins of the prior art, however, are insoluble in water. A small quantity of water can be used or mixed with the prior radiation-curable resins, but a large quantity of water cannot be used. The mere addition of water therefore is not a viable solution. Hence, it presently is a prime goal to develop a water-based radiation-curable resin as a chemical solution to overcome prior unsolved problems.
Japanese Laid-Open Patent Publication No. SHO-62-22816 discloses a water-based ionomer urethane acrylate, and its method of preparation, which was developed to solve the problems of prior art resin compositions. In preparing the ionomer urethane acrylate, a mono-hydroxyl group-containing compound containing a polymerizable unsaturated group first is reacted with an organic polyisocyanate. Then, the residual isocyanate groups are reacted with a polyol compound. Next, an organic solvent, such as acetone, is added, and a carboxyl group is introduced by reacting mono-amino-alkyl carboxylic acid in the presence of water to effect conversion into salt. The resulting product is dispersed in water to provide an emulsion. Finally, the organic solvent is removed under reduced pressure. An ionomer urethane acrylate is prepared by this method.
The above-described method of production generally has the following disadvantages:
The above-mentioned disadvantages subject the polymerizable unsaturated groups introduced into the urethane to destabilizing conditions. The destabilizing conditions can induce gelation due to polymerization of the unsaturated group, and pose a serious problem from a viewpoint of industrial production. The problem possibly can be alleviated by either using a powerful inhibitor selected from various commonly-used inhibitors, or using a large quantity of an inhibitor. When such a method is used, however, the rate of polymerization during radiation processing is reduced. This, in turn, reduces the overall processing speed, and creates a new problem.
The present invention solves problems encountered in the prior art. The present invention is directed to providing radiation-curable polyurethane emulsion compositions that do not include solvent, and that form a cured film which is not brittle and has a good adhesion to a substrate.